In the refining of petroleum products, such as crude oil, hydrochloric acid is generated which can cause high corrosion rates on the distillation unit metallurgy, including the overhead system. Neutralizing amines are added to the overhead system to neutralize the hydrochloric acid (HCl) and make it less corrosive. However, excess amines can form salts that will also lead to corrosion. Consequently, the refining industry has, for many years, suffered from amine-hydrochloride salt deposition in crude oil distillation towers, overhead and pumparound circuits. The problem occurs when ammonia and/or amines are present in the desalted crude. These amines react with hydrochloric acid and other acids while ascending the crude tower and deposit as corrosive salts in the tower and the top pumparound equipment. The amines can be present from several sources, including but not necessarily limited to, crude oil (e.g. hydrogen sulfide (H2S) scavenger chemicals—amines added to neutralize the corrosive and other deleterious effects of H2S), slop oil (frequently containing gas scrubbing unit amines) and desalter wash water (often composed of overhead sour water containing amine neutralizer). The problem has worsened in recent years in part due to higher crude salt content, which yields higher HCl contents as a by-product and in turn requires more overhead neutralizer, consequently both salt reactants are present in higher quantities. Additionally, market conditions have encouraged many crude towers to be operated at a colder top temperature, which further encourages salt formation in towers. Longer run cycles between turnarounds have caused the problem to become a priority. Clearly, amine salting in towers has become a bigger problem in recent years, and future trends indicate continuation of the problem.
In a specific instance, a unit has an excessive level of ammonia that contributes to salt formation and the operators are processing above design so that the stream velocities are too high to use a water wash (a common remedy for salts) without experiencing velocity-accelerated corrosion. In a second specific example the operators desire to process a crude oil with a tramp amine. The use of an acid upstream at the desalter reduces the amine to a level that does not form a salt, but the cost of the acid treatment is high.
Solutions examined thus far fall into two categories. First, for cases where the amine is coming in with the crude oil or slop oil, the primary option is to segregate the offending streams and keep them out of the crude unit. This approach is economically unattractive in many cases. Second, in cases where the problem occurs due to recycle of overhead neutralizer by use of the distillation overhead water as a desalter wash source, the approach has been to switch to overhead amines that will not form a salt at tower conditions or use another desalter wash source. These techniques are also economically unattractive in most applications, since these alternative neutralizers cost from three to four times as much as the conventionally used amines.
Additional changes are foreseen which are likely to make the problem even worse. The economic incentive to use discounted crudes has led to a general deterioration of crude quality, and further, more plants are attempting to maximize internal water reuse. A recent effort to design new amine neutralizer options for overhead systems does not offer relief in all cases, because such amines will not help in systems where salts are present from ammonia or tramp amines entering the system with crude oil or slop oil.
It would be desirable if methods and/or compositions could be devised that would reduce, alleviate or eliminate corrosion caused by undesired amine salts where amines enter refinery towers and at other locations.